As typical ink discharge systems, there are a method for allowing a current to flow through an electric resistor arranged in a pressurizing chamber, vaporizing and expanding water in an ink through generated heat, applying a pressure to the ink, and discharging the ink and a method for forming part of a channel member constituting the pressurizing chamber as piezoelectric bodies or installing the piezoelectric bodies on the channel member, selectively driving the piezoelectric bodies associated with a plurality of nozzles, thereby deforming the pressurizing chamber based on dynamic pressures of the respective piezoelectric bodies, and then discharging a liquid from the nozzles.
In recent years, applications of inkjet to industrial purposes have expanded, and not only waterborne inks but also various kinds of inks such as a solvent ink or a UV cure ink have been used. The method for deforming a pressurizing chamber based on a dynamic pressure of each piezoelectric body and discharging an ink is superior in compatibility with the various kinds of inks, and it has been used in many fields.
On the other hand, in recent years, to realize density growth, an interval between neighboring nozzles is becoming narrower in an inkjet head. In an inkjet head having a configuration that partition walls having piezoelectric bodies are installed on a channel member and the partition walls and pressurizing chambers are alternately aligned, for example, an interval between nozzles with nozzle density of 180 dpi (dpi in the present invention represents the number of dots per 2.54 cm) is 140 μm. In this case, if the partition wall of 70 μm is present in the interval of 140 μm, a width of an ink channel is 70 μm. Assuming that nozzles have a shape that spreads from an outer surface toward an ink channel side along a nozzle plate thickness direction and a diameter on the ink channel side is 40 μm, a length from an end portion of the partition wall to the nozzle is only 15 μm. At the time of bonding the nozzle plate to, e.g., a recording element substrate or the channel member, when a heating treatment for hardening the adhesive is carried out, since viscosity of the adhesive is lowered due to this heating operation and the adhesive tends to flow, the adhesive flows into nozzle portions that are close to each other, and part of the nozzle portions or, in the worst case, the entire nozzle portions may be eventually closed. Especially, when an ink to be adopted has characteristics that swell or dissolve the adhesive, ink resisting properties of the adhesive must be enhanced. Therefore, although it is effective to select an adhesive whose glass-transition point increases after hardened, heat the adhesive at a high temperature at the time of hardening it, and raise the glass-transition point of the adhesive, viscosity of the adhesive before hardened is further lowered when the adhesive is heated to a high temperature, and the nozzle portions are more likely to be closed.
In view of preventing the adhesive from flowing into such a nozzle region, when an application amount of the adhesive is reduced, irregularities on a surface of the channel member or foreign particles adhering to this surface may produce a void where the adhesive is not present between the nozzle plate and the channel member, and the ink may leak through this void in some cases. On the other hand, when an adding amount of the adhesive is increased, an amount of the adhesive protruding from a gap between members increases, the adhesive flows into the nozzle region, and it is difficult to eliminate both the void and the flow into the nozzles.
As described above, according to the system that at least part of the channel member is formed of piezoelectric body and the piezoelectric body is deformed by an electrical signal to apply a pressure to the ink, an electrode configured to apply a voltage to the piezoelectric body is installed on the piezoelectric body surface. When the ink comes into contact with the electrode, since the electrode corrodes and comes down, a protective film is often provided on the electrode in order to protect it. The protective film can be formed by, e.g., heating and evaporating paraxylylene or its derivative in a vacuum chamber and polymerizing a radical, which has been generated by thermal decomposition, on the surface of the piezoelectric body having the electrode installed in the vacuum chamber. Since less pinholes are provided and the protective film can be formed to the inner side in a complicated structure, the protective film formed by this technique is often used.
However, the surface of the protective film formed by the above method has irregularities, and they may be an obstacle in subsequent processing. In particular, in case of bonding the nozzle plate to the surface of the protective film, besides the problem that the adhesive flows into the nozzles or a void is produced in the adhesive layer, there is also a problem that the nozzle plate is bonded in a curved state due to the irregularities, a discharge angle of droplets is thereby changed, and so-called bend occurs.
Patent Document 1 discloses that, to avoid the bend of discharge due to irregularities on a nozzle plate, many protrusions having the same height are provided on a joint surface of the nozzle plate for a head main body, and a thickness of an adhesive layer is uniformed when the nozzle plate is bonded to the head main body using the adhesive. However, the irregularities of the protective layer are disorganized, the protrusions may be formed on a convex portion of the protective film in the method according to Patent Document 1, and hence heights of the protrusions cannot be made constant. Further, provision of such protrusions requires many manufacturing steps, which is not preferable in terms of production efficiency.
On the other hand, Patent Document 2 discloses a method for producing an inkjet head by which an adhesive is applied to an upper surface of a protection layer having irregularities, the adhesive is hardened, a flattened layer is provided, the adhesive is further applied to the upper side of the flattened layer, and a nozzle plate is bonded. However, the process is complicated, and an effect for avoiding the bend of discharge provided by flattening is not sufficient.